1. Field of the Invention
The present invention relates to a nuclear fuel assembly grid which simplifies insertion of a plurality of fuel rods into a plurality of grid cells during the assembly of the nuclear fuel assembly.
2. Description of the Prior Art
A construction as shown in FIG. 24 of the attached drawings is known as an example of a nuclear fuel assembly which is mounted in a pressurized water reactor.
In FIG. 24, top and bottom nozzles 1 and 2 are vertically spaced and arranged in facing relation to each other. A plurality of control-rod guide thimbles 3 are fixed to and extend between the top and bottom nozzles 1 and 2. A plurality of grids 4 are mounted at intermediate portions of the respective control-rod guide thimbles 3 in a vertically spaced relation to each other.
As shown in FIG. 25, each of the grids 4 is formed such that a plurality of straps 5, each in the form of a thin plate, are assembled perpendicularly to each other into a grid by a mutual fitting of slits which are formed in the straps 5 in longitudinally equidistantly spaced relation to each other. A plurality of grid cells 6 are defined in each of the grids 4. A pair of dimples 8 and a pair of springs 9 for supporting a fuel rod 7 are mounted on the wall surface of each of the grid cells 6 in facing relation to each other. The fuel rod 7 inserted in the grid cell 6 is supported by being pushed against the dimples 8 by the springs 9.
An assembly method for the nuclear fuel assembly constructed above will next be described.
First, the grids 4 are arranged in a spaced relation to each other with a predetermined spacing. Then the control-rod guide thimbles 3 are inserted into and fixed to a part of the corresponding grid cells 6 of each of the grids 4. Subsequently, the fuel rods 7 are inserted into corresponding grid cells 6 in each of the grids 4 which are supported by the control-rod guide thimbles 3, with the fuel rod 7 in sliding contact with the dimples 8 and the springs 9. In this manner, the fuel rods 7 are held in fixed arrangement in the grid cells 6 by the dimples 8 and the springs 9. After insertion of all of the fuel rods 7, the top and bottom nozzles 1 and 2 are fixed to respective opposite ends of the control-rod guide thimbles 3.
A problem with the aforesaid assembling method of the nuclear fuel assembly is that when the fuel rods 7 are inserted into the respective grid cells 6 in the grids 4, the outer periphery of each of the fuel rods 7 is clamped between the dimples 8 and the springs 9, and insertion of the fuel rod 7 into the corresponding grid cells 6 is restricted by the resilient force of the springs 9. Thus, difficulties arise in the insertion operation detracting from the working or operating efficiency.